Powder coating for packaging containers

ABSTRACT

The present invention relates to a powder coating for application to packaging containers, 
     1) the powder coating comprising 
     A) at least one thermoplastic, 
     B) if desired, at least one thermoset, preferably an epoxy resin having an epoxy equivalent weight of from 400 to 3000, 
     C) if desired, pigments, fillers, catalysts, typical powder coating additives such as degassing agents, levelling agents, free-radical scavengers and antioxidants, and 
     2) the powder coating having a particle-size distribution such that 
     a) at least 90 percent by mass of the powder-coating particles have a size of between 1 and 120 μm, 
     b) the maximum size of the powder-coating particles is ≦150 μm for at least 99 percent by mass of the particles, and 
     c) the mean size of the powder-coating particles in between 1 and 60 μm, preferably from 1 to 30 μm.

The present invention relates to a powder coating which is particularlysuitable as a coating for packaging containers.

Liquid coating materials are nowadays preferably used for the coating ofpackaging containers. These materials give rise to numerousenvironmental problems as a result of their solvent content. This alsoapplies to cases where aqueous coating materials are employed.

It is therefore increasingly being attempted to replace these coatingmaterials by low-solvent or solvent-free substitutes. For example,thermoplastic powder coatings have already been used frequently for thecovering of can weld seams. These products are prepared by expensivecold milling from the corresponding thermoplastics.

Furthermore, EP-B 119 164 discloses thermosetting powder coatings forthe weld-seam covering of metal containers which are used to hold foodsor beverages. The binder in these thermosetting powder coatings is amixture of an aromatic epoxy resin having on average not more than 2epoxide groups per molecule and of an aromatic epoxy resin having onaverage more than 2 epoxide groups per molecule. The hardener used isthe condensation product of bisphenol A diglycidyl ether with bisphenolA, and/or an acidic polyester based on trimellitic anhydride/aliphaticpolyol. In EP-B-119 164, however, there are no indications regardingappropriate particle size and particle-size distributions of the powdercoatings. A further disadvantage is that these powder coatings aresuitable only for weld-seam covering.

EP-B-10 805 discloses powder coatings for the interior coating of cans,containing a polyester having terminal carboxyl groups and an OH numberof less than 10 mg of KOH/g, and an epoxy resin. As curing catalystthese powder coatings contain choline derivatives. The powder coatingshave a mean particle size of between 20 and 150 μm. However, EP-B-10 805contains no information on how can interior coatings can be obtainedthat give coherent films even at coat thicknesses ≦15 μm. Moreover, as aresult of the low OH number of the polyester, these powder coatings havethe disadvantage of only poor crosslinking. Correspondingly, this systemshown drying times which are unacceptable in practice, raging from 10 to40 minutes at from 150 to 220° C., whereas the drying time in modernproduction plants is at maximum 20 to 30 s at an article temperature offrom 260 to 280° C.

U.S. Pat. No. 4,497,837 discloses powder coatings for the interiorcoating of cans and can lids, containing an epoxy resin and aromaticamines, Lewis acids or acid anhydrides as hardeners. The powder coatingshave a mean particle size of between 20 and 150 μm, preferably from 30to 70 μm. A disadvantage with these systems is the high minimum coatthickness of 38 μm in order to achieve coatings without excessiveporosity. Furthermore, these powder coatings have the disadvantage thatoven residence times of between 5 and 12 minutes are necessary in orderto cure the systems described.

Furthermore, U.S. Pat. No. 3,962,486 discloses powder coatings for theinterior coating of cans that likewise include an epoxy resin andaromatic amines, epoxy-amine adducts or acid anhydrides. By using theplasma spray coating technique it is possible to produce coatings whichmeet the usual requirements placed on interior coatings of foodpackaging even at low coat thicknesses of less than 13 μm. To ensure theability for application by the plasma spray technique, the only powdercoatings which it is permitted to use are those having a maximumparticle size ≦100 μm and a sufficiently low melt viscosity.

The use of amine hardeners, however, leads to inadequate sterilizationresistance in the resulting coatings. Further disadvantages are thatepoxy resins cured with amines tend toward embrittlement and have verypoor elasticities. Acid anhydride hardeners have the disadvantage thatthey are highly irritant, with the result that particular precautionarymeasures are necessary when formulating the powder coatings.

Finally, U.S. Pat. No. 4,183,974 discloses powder coatings for theinterior coating of cans, containing an epoxy resin and an aminehardener. These powder coatings have mean particle sizes of between 1and 100 μm, preferably between 1 and 10 μm. The resulting coatings doindeed have the required low porosity even at coat thicknesses of ≦13μm; however, the resulting coatings are in need of improvement. Furtherdisadvantages are that epoxy resins cured with amines tend towardembrittlement and have very poor elasticities.

Coatings produced from powder coating materials for preserve cans,moreover, are also known from German Patent Applications P 40 38 681.3and P 42 04 266.6.

The results with powder coatings have overall not been satisfactory todate; in particular, increased coat thicknesses are required in order toobtain a uniform appearance. Furthermore, when so-called double canswhich are produced for preserve cans are broken open, the broken edgesof the coating material are unsatisfactory.

The object of the present invention, therefore, is to provide a powdercoating for the interior coating of packaging containers, especiallypreserve cans, which no longer has the disadvantages set out in theintroduction.

The object is achieved in that

1) the powder coating comprises

A) at least one thermoplastic,

B) if desired, at least one thermoset, preferably an epoxy resin havingan epoxy equivalent weight of from 400 to 3000,

C) if desired, pigments, fillers, catalysts, typical powder coatingadditives such as degassing agents, levelling agents, free-radicalscavengers and anti-oxidants, and

2) the powder coating has a particle-size distribution such that

a) at least 90 percent by mass of the powder-coating particles have asize of between 1 and 120 μm,

b) the maximum particle size of the powder-coating particles is ≦150 μmfor at least 99 percent by mass of the particles, and

c) the mean size of the powder-coating particles is between 1 and 60 μm,preferably 1 and 30 μm.

The proportion of component A) in accordance with the invention is from60 to 90, preferably from 70 to 80% by weight, that of component B) from0 to 20, preferably from 10 to 15% by weight, and that of component C)from 10 to 20, preferably from 10 to 15% by weight.

The invention also relates to a process for the interior coating ofpackaging containers in the case of which these powder coatings areapplied.

Finally, the invention also provides for the use of the powder coatingsfor the interior coating of packaging containers.

It is surprising and was not foreseeable that particle sizes in therange <30 μm can be achieved with the abovementioned thermoplasticpowder coatings by using the abovementioned mixture with thermosets, andthat the set of properties and therefore the intended use of powdercoatings can be controlled in a targeted manner by establishing aspecific particle-size distribution. At the same time, the novel powdercoatings can be cured rapidly, are easy to handle and are simple toapply.

The novel powder coatings are additionally notable for the fact thatcoatings having only a very low coat thickness of ≦15 μm have theproperties desired for interior coatings by the can manufacturers. Inparticular, these coatings have the required low porosity even at a lowcoat thickness of ≦15 μm. These coatings are additionally notable forgood adhesion, high flexibility and good pasteurization andsterilization resistance.

Thermoplastics which can be used in accordance with the invention arepolymers, copolymers, terpolymers, graft polymers and ionomers. Exampleswhich may be mentioned are polyurethanes, polyamides, polyethylenes andpolyesters.

In accordance with the invention it is possible in particular to employpolyurethane-based plastics containing one or more polyester diolshaving a molecular weight from 600 to 1200, preferably from 800 to 1000.Particularly suitable polyester diols are polybutanediol adipates,polyethylene glycol or mixtures thereof.

To prepare the thermoplastic polyurethanes the structural components arereacted in appropriate amounts in the presence, if desired, ofcatalysts, auxiliaries and/or additives.

Suitable catalysts, which in particular accelerate the reaction betweenthe NCO groups of the diisocyanates and the hydroxyl groups of thestructural components, are the tertiary amines which are customary andknown in the prior art, for example triethylamine,dimethylcyclohexylamine, N-methylmorpholine, N,N'-dimethylpiperazine,2-(dimethylaminoethoxy)ethanol, diazabicyclo[2.2.2]octane and the like,and, in particular, organometallic compounds, such as titanium acidesters, iron compounds, for example iron(III) acetylacetonate, tincompounds, for example tin diacetate, tin dioctoate, tin dilaurate orthe tin dialkyl salts of aliphatic carboxylic acids such as dibutyltindiacetate, dibutyltin dilaurate or the like. The catalysts are commonlyemployed in amounts of from 0.002 to 0.1 part per 100 parts ofpolyhydroxy compound.

In addition to catalysts, auxiliaries and/or additives can also beincorporated into the thermoplastics (A). Mention may be made, forexample, of lubricants, inhibitors, stabilizers against hydrolysis,light, heat or discoloration, dyes, pigments, anti-oxidants and/or freeradical scavengers, reinforcing agents and plasticizers.

Further details about the abovementioned auxiliaries and additives aregiven in the technical literature, for example in the monograph by J. H.Saunders and K. C. Frisch "High Polymers", Volume XVI, Polyurethanes,Part 1 and 2, Verlag Interscience Publishers 1962 and 1996, from theabovementioned Kunststoff-Handbuch, Volume VII, Polyurethanes or fromDE-A-29 01 774.

It is preferred to employ thermoplastic polyurethanes having a Shore Ahardness of from 70 to 95 and those having a Shore D hardness of from 54to 80 which are prepared by reacting polyoxytetramethylene glycol oralkanediol polyadipates having 2 to 6 carbon atoms in the alkyleneradical, linear aliphatic and/or cycloaliphatic diisocyanates, forexample hexamethylene 1,6-diisocyanate or 4,4'-dicyclohexylmethanediisocyanate, and/or aromatic diisocyanates, for example4,4'-diphenylmethane diisocyanate and 1,4-butanediol in a ratio ofequivalents of NCO:OH groups of from 1:0.95 to 1:0.5.

In accordance with the invention, particular preference is given topolyethylene compounds which can be obtained, for example, under thedesignation Lupolen® (obtainable from BASF AG). Polyethylenes of thiskind can be prepared by free-radical polymerization of ethylene at highpressures (from 1500 to 3000 bar) or by coordinative polymerization withthe aid of catalysts at low pressures. Depending on the polymerizationconditions, the polymers formed are variable in density (from 0.90 to0.97 g/cm³) and in molar mass. Polyethylenes are normally characterizedby means of densities and melt indices. By polymerizing ethylene withpolar monomers such as vinyl acetate, acrylates, acrylic acid or apolarα-olefins such as 1-butene, 1-hexene, etc., it is possible to obtaincopolymers with specific changes in polymer structure. Preference isgiven in accordance with the invention to those polyethylenes formed bylow-pressure polymerization, such as, for example, the above-citedLupolen.

The polyethylenes are partially crystalline plastics. Depending on thepolymerization conditions, polyethylenes with a variable degree ofbranching are produced. The less branched the macromolecules, the higherthe crystalline fraction and therefore the density as well. The level ofthe crystalline fraction and the cryetallite thicknesses determine themelting properties, i.e. the melting point and the heat of fusion of thepolyethylenes. The mechanical properties depend directly on thecrystallinity and density and on the molar mass. Rigidity and hardnessincrease as the density rises. In the case of the copolymers, rigidityand hardness decrease as the comonomer content grows, an a result offalling crystallinity. Consequently, in accordance with the inventionthe degrees of branching and the crystalline fraction are to becontrolled such that the preferred hardness ranges are reached.

The copolymers, terpolymers, graft copolymers and ionomers that can beemployed in accordance with the invention can be used under the provisothat they have carboxyl or anhydride groups, or groups which can behydrolyzed to carboxyl groups, and that the melt index of the polymers,measured at 190° C. under a load of 2.16 kg, is between 0.1 and 30 g/10min, preferably between 0.2 and 25 g/10 min and, with particularpreference, between 0.5 and 20 g/10 min.

Suitable co- and terpolymers can be prepared by copolymerizing ethyleneand α,β-unsaturated carboxylic acids such as acrylic acid, methacrylicacid, itaconic acid, crotonic acid, isocrotonic acid, maleic acid andfumaric acid, the corresponding anhydrides or the corresponding estersor monoesters having 1 to 8 carbon atoms in the alcohol residue, such asmethyl, ethyl, propyl, butyl, pentyl, hexyl, cyclohexyl, heptyl, octyland 2-ethylhexyl esters of the acids listed. It is likewise possible toemploy the corresponding salts of the carboxylic acids listed an well,for instance the sodium, potassium, lithium, magnesium, calcium, zincand ammonium salts. Preference is given to employing the carboxylicacids and their anhydrides.

In the course of the copolymerization it is also possible to employfurther monomers which can be copolymerized with ethylene and theunsaturated carbonyl compounds. Suitable examples are α-olefins having 3to 10 carbon atoms, vinyl acetate and vinyl propionate.

In this context, the amounts of the monomers employed are chosen suchthat the corresponding polymer has a carboxyl group content of from 0.1to 30% by weight, preferably from 2 to 20% by weight, and that thecontent of ethylene units in the polymer is up to 99.9% by weight,preferably between 75 and 95% by weight.

Suitable graft copolymers can be prepared by grafting at least onepolymer from the group of the polyolefins with up to 10% by weight,preferably up to 5% by weight, based on the overall weight of themonomers, of at least one monomer from the group of the α,β-unsaturatedcarboxylic acids, their anhydrides, their esters or salts, in thepresence or absence of peroxides.

The ionomers employed can be prepared by the copolymerization, alreadydescribed above, of ethylene and, if desired, other monomers with saltsof α,β-unsaturated carboxylic acids or by partial neutralization of thecarboxylic acid-containing copolymers, terpolymers and graft polymers,already described above, with salts, oxides and hydroxides of sodium,potassium, lithium, magnesium, calcium, zinc and ammonium.Neutralization can be carried out in the melt or in the solution. Inthis context, the amount of basic compound is chosen such that thedegree of neutralization of the polymer is between 0.1 and 99%,preferably between 0.1 and 75% and, with very particular preference,between 0.1 and 40%.

The thermosets (B) that are employed in the novel powder coatings arepreferably epoxy resins, in particular in combination with polyethylenesor polyesters. They are in particular solid epoxy resins having anepoxide equivalent weight of from 400 to 3000, preferably from 600 to8000, particularly preferably 700 to 800. Aromatic, aliphatic and/orcycloaliphatic epoxy resins are suitable. Preference is given to the useof aromatic epoxy resins based on bisphenol A and/or bisphenol F and/orepoxy resins of the novolak type. Particularly preferred epoxy resinsemployed based on bisphenol A or bisphenol F have an epoxide equivalentweight of from 500 to 2000. Particularly preferred epoxy resins employedof the novolak type have an epoxide equivalent weight of from 500 to1000. In this context, epoxy resins based on bisphenol A and/orbisphenol F generally have a functionality of not more than 2, and epoxyresins of the novolak type a functionality of in general at least 2.However, the epoxy resins based on bisphenol A and/or bisphenol F mayalso be brought to a functionality of more than 2 as a result ofbranching, for example by means of trimethylolpropane, glycerol,pentaerythritol or other branching reagents.

It is of course also possible to employ other epoxy resins, for examplealkylene glycol diglycidyl ethers or their branched follow-on products,or epoxy resins based on bisphenol A or F, or the like, that areflexibilized with alkylene glycols. Also suitable, furthermore, aremixtures of various of the epoxy resins mentioned.

Examples of suitable epoxy resins are the products obtainablecommercially under the following name: Epikote® 154, 1001, 1002, 1055,1004, 100.7, 1009, 30034F-10 from Shell-Chemie, XZ 86 795 and DER® 664,667, 669, 662, 642U and 672U from Dow, and Araldit®, GT 6064, GT 7072,GT 7023, GT 7004, GT 7304, GT 7097 and GT 7220 from Ciba Geigy.

In the thermoset components (B) of the powder coatings it is possiblefor hardener components to be present, or to be added thereto. They arenormally employed in an amount of from 10 to 50% by weight, preferablyfrom 15 to 40% by weight, based in each came on the overall weight ofcomponent (B).

Compounds suitable as hardener component are all solids having more thanone phenolic OR group, preferably from 1.8 to 4 and, with particularpreference, ≦3 phenolic OH groups per molecule, and having ahydroxyl-equivalent weight, based on OH groups, of from 100 to 500,preferably from 200 to 300.

Hardeners preferably used are those based on bisphenol A and/orbisphenol F. A particularly preferred hardener is the condensationproduct of the diglycidyl ether of bisphenol A or bisphenol F withbisphenol A or bisphenol F, respectively, in particular the condensationproduct having an equivalent weight, based on phenolic hydroxyl groups,of from 220 to 280. These condensation products are usually prepared byreacting generally excess bisphenol with a bisphenol diglycidyl ether inthe presence of an appropriate catalyst. The condensation product ispreferably prepared by reacting the diglycidyl ether with the bisphenolin a weight ratio of from 0.5 to 2. These hardeners on the basis ofthese condensation products of the bisphenol diglycidyl ether with abisphenol generally have a functionality of not more than 2, it beingpossible in turn to establish higher functionalities by using branchingreagents.

Furthermore, other suitable hardeners are the reaction products ofbisphenols with epoxy resins of the novolak type. These hardeners arepreferably obtained by reacting the epoxy resin with the bisphenol in aweight ratio of from 0.5 to 2 in the presence of an appropriatecatalyst.

Suitable examples are the phenolic hardeners described in DE-C-23 12 409in column 5, line 2 to column 6, line 55.

Furthermore, it is also possible to employ the phenolic hardenersdescribed in DE-A-30 27 140.

Flexibilized hardeners and/or hardeners modified with branching reagentsare of course also suitable. In addition, it is possible as well to usemixtures of various hardeners of those mentioned above. Among these,preference is given to the use of FDA-approved hardeners.

Examples of such commercially available, hydroxyl-containing hardenerswhich are preferably employed are the products obtainable commerciallyunder the following names: Harter XB 3082 from Ciba Geigy and Epikure®169 and 171 from Shell-Chemie.

As a further component the novel powder coatings can include at leastone curing catalyst, normally in an amount of from 0.01 to 5.0% byweight, preferably from 0.05 to 2.0% by weight, based in each case onthe overall weight of the powder coating.

The catalyst is advantageously imidazole, 2-methylimidazole,ethyltriphenylphosphonium chloride or another salt thereof, a quinolinederivative as described, for example, in EP-B-10 805, a primary,secondary or tertiary aminophenol, aluminum acetylacetonate or atoluenesulfonic acid salt, or a mixture of various of the abovementionedcatalysts.

The commercially obtainable hydroxyl-containing hardeners normallyalready contain a curing catalyst.

Furthermore, the thermoset powder coatings according to component (B)may, if desired, include auxiliaries and additives. Examples of theseare the substances listed for component (A).

In addition, the novel powder coatings with the components (A) and (B)may also comprise from 0 to 55% by weight, preferably from 15 to 25% byweight, of fillers. FDA-approved fillers are preferably employed.Inorganic fillers are generally employed, for example titanium dioxide,such as Kronos 2160 from Kronos Titan, Rutil R 902 from Du Pont and RC566 from Sachtleben, barium sulfate and silicate-based fillers, forexample talc, kaolin, magnesium aluminum silicates, mica and the like.Preference is given to titanium dioxide and fillers of the quartz sandtype.

The novel powder coatings with the components (A) and, if desired, (C)may, if desired, additionally contain from 0.01 to 10% by weight,preferably from 0.1 to 2% by weight, based on the overall weight of thepowder coating, of further auxiliaries and additives. Examples of theseare levelling agents, flow aids, deaerating agents, for example benzoin,pigments, or the like.

Powder coatings particularly suitable for use as an interior coating ofpackaging containers are those with the components (A), optionally (B)and optionally (C), having both only a small proportion of very fineparticles (particle size <5 μm) and also, at the same time, only a verysmall proportion of coarse powder-coating particles (particle size a ≧25μm), i.e. those having a particle-size distribution which is as narrowas possible.

For use for the interior coating of packaging containers, theparticle-size distribution is generally established so that at least 90percent by mass of the powder-coating particles have a size between 1and 120 μm. Preferably, 90 percent by mass of the powder-coatingparticles have a size between 1 and 40 μm and, with particularpreference, between 5 and 25 μm. The maximum size of the powder-coatingparticles is ≦150 μm for at least 99 percent by mass of the particles,preferably ≦60 μm and, with particular preference, <40 μm. The mean sizeof the powder-coating particles is between 1 and 30 μm, particularlypreferably between 2 and 12 μm. The coatings themselves have therequired low porosity at a low coat thickness of ≦15 μm. Furthermore,these coatings are notable for good adhesion, high flexibility and goodpasteurization and sterilization resistance.

In addition, it is essential to the invention that, when the powdercoatings are used for the interior coating of the packaging containers,the particle-size distribution is adjusted such that the slope S of theparticle distribution curve at the point of inflexion is ≧100,preferably ≧150 and, with particular preference, ≧200. To obtaincoatings having particularly good properties, it in very particularlypreferred to employ powder coatings where the slope S of theparticle-size distribution curve at the point of inflexion is ≧300.However, the production costs of the powder coatings increase greatly asthe slope increases.

Here, the slope S is defined as the limit value for f(x₂)-f(x₁) towardzero of (f(x₂)-f(x₁))/1 g ((x₂ /x₁)) at the point of inflexion of theparticle distribution curve. This particle distribution curve representsthe plot of the cumulative percentages by mass (f(x)) against theabsolute particle diameter (x), the particle diameter being representedon the logarithmic scale and the cumulative percentages by mass on thelinear scale. For use as an interior coating of packaging containers,therefore, particularly suitable powder coatings are those having bothonly a relatively small proportion of very fine particles (size <5 μm)and, at the same time, only a very small proportion of coarsepowder-coating particles (particle size >25 μm), i.e. having aparticle-size distribution which is as narrow as possible.

For the use as a weld-seam cover the particle size distribution (b) isadjusted so that at least 90 percent by mass of the powder-coatingparticles have a size of between 1 and 100 μm. Preference is given tothe use of powder coatings for which at least 90 percent by mass of thepowder-coating particles have a size of between 5 and 100 μm. Themaximum size of the powder-coating particles is ≦150 μm for at least 99percent by mass of the particles, preferably ≦100 μm. The mean size (c)of the powder-coating particles is preferably between >20 and 60 μm,particularly preferably between 25 and 40 μm. For use for weld-seamcovering, therefore, the powder coatings employed for the interiorcoating of the packaging containers are also suitable in principle.However, for use for weld-seam covering it is preferred to employ powdercoatings containing a higher proportion of coarse powder-coatingparticles.

Finally, the present invention also relates to a process for preparingthe described powder coatings for the coating of packaging containers.

The preparation of the solid powder coatings with the components (A)and, if used, (B) and also, if used, (C) is carried out by known methods(cf. e.g. product information from BASF Lacke+Farben AG, "Pulverlacke"[powder coatings], 1990) by homogenizing and dispersing, for example bymeans of an extruder, screw compounder, and the like. Following thepreparation of the powder coatings, they are prepared for dispersion bymilling and, if desired, by classifying and sieving. The spray-dryingtechniques also come into consideration.

For the preparation of the thermoplastics a polyester or a polyurethaneor a polyethylene is milled so that at least 90 percent by mass of thepowder-coating particles have a size of between 1 and 120 μm, themaximum size of the powder-coating particles is ≦150 μm for at least 99percent by mass of the particles, and the mean size of thepowder-coating particles is between 1 and 34 μm. Particular preferenceis given to particle sizes where at least 90 percent by mass of thepowder-coating particles have a size of between 1 and 40 μm, the maximumsize of the powder-coating particles is ≦60 μm for at least 99 percentby mass of the particles, and the mean size of the powder-coatingparticles is between 2 and 12 μm. Very particular preference is given toparticle sizes where at least 90 percent by mass of the powder-coatingparticles have a size of between 5 and 25 μm, the maximum size of thepowder-coating particles is ≦40 μm for at least 99 percent by mass ofthe particles, and the mean size of the powder-coating particles isbetween 2 and 12 μm.

The thermosets are prepared in the same way. In accordance with theinvention, they can be milled together with the thermoplastics orseparately.

Milling is followed, in accordance with the invention, by the additionif desired of pigments, fillers, catalysts, typical powder coatingadditives such as degassing agents, levelling agents, free-radicalscavengers and antioxidants. This is preferably done in amounts of from15 to 10% by weight.

The packaging containers which are coated with the novel powder coatingscan consist of a very wide variety of materials, can have a very widevariety of sizes and shapes, and can have been prepared by variousmethods. In particular, however, the novel powder coating dispersionsare used to coat metallic containers. These metal containers can havebeen prepared by first of all rolling sheet metal and then joining it byfolding back the edge. The endpieces can then be fastened to theresulting cylinder. The novel powder coatings are employed both forcovering the weld seam and for the interior coatings of the can bodies,which in general already have a base. Furthermore, deep-drawn metalcontainers can also be coated internally with the novel powder coatings.The powder coatings are, however, of course also suitable for thecoating of can lids and can bases.

The packaging containers can consist of a wide variety of materials, forexample aluminum, black plate, tinplate and various ferrous alloys,which may have been given a passivating layer based on compounds ofnickel, of chromium and of tin. Containers of this kind are commonlyused as containers for foods and beverages, for instance for beer,juices, fizzy drinks, soups, vegetables, meat dishes, fish dishes, andalso, for example, for animal food.

Application takes place in accordance with known methods as aredescribed, for example, in U.S. Pat. No. 4,183,974. In this context, thepowder-coating particles are electrostatically charged by friction(triboelectricity). The powder-coating particles are applied with theaid of special spray heads which are known to the skilled worker.

For the interior coating of the packaging containers, the powdercoatings are usually applied in a coat thickness ≦15 μm, preferably from10 to 14 μm. Even at these low coat thicknesses, the coatings meet therequirements commonly placed on such films. The powder coatings can ofcourse also be applied at higher coat thicknesses. For the covering ofweld seams, the powder coatings are usually applied in a coat thicknessof ≦200 μm, preferably ≦80 μm.

The packaging container whose weld seam or interior has been providedwith the novel powder coating is subsequently subjected to a heattreatment in order to cure the powder coating. This heat treatment canbe carried out in a variety of ways. In practice, the containers areoften conveyed through a through-type oven for this purpose. In such anoven the powder coatings are generally cured fully at containertemperatures between 180-240° C. within a period of 5-15 s. In this caseit is possible for the through-type oven to be operated at constanttemperature or to have a temperature profile which is set in accordancewith the prevailing circumstances.

In the case of use for weld seam covering, the novel coatings have ahigh flexibility, so that the weld seam covering is able to followdeformations of the packaging container in the course of furtherprocessing without becoming detached or cracking. Further advantages arethat good sterilization resistance is achieved and, in the case of usefor double cans, the broken edges of the coating material can beadjusted in a targeted manner.

In accordance with the invention, the powder-coating layer canadditionally be coated with a plastics film, preferably with apolypropylene top layer.

The polypropylenes used are random polypropylene copolymers, in eachcase in the form of a film. These may be composite films which areobtained, for example, by coextrunsion of different random polypropylenecopolymers. Polypropylene films of this kind are produced by knownmethods (blow molding, chill-roll techniques, etc.) from granules of thepolypropylenes.

In accordance with the invention, random polypropylene copolymerssuitable for preparing the polypropylene films of the polypropylenefilm/adhesion promoter/metal composites are those obtained by randomcopolymerization of from 90 to 99% by weight, preferably from 93 to 99%by weight, of propylene and from 1 to 10% by weight, preferably from 1to 7% by weight, based in each case on the overall monomer weight, ofcomonomers. The random copolymers have a molar mass distribution M_(w):M_(n) in the range from 2 to 10, preferably 3 to 6, and a melt indexMFI 230° C./2.16 kg in the range from 1 to 20 g/10 min, preferably inthe range from 4 to 15 g/10 min (measured in accordance with DIN 53735). Polypropylenes of this kind and methods for their preparation areknown. They can be prepared, for example, by the polymerizationtechnique described in DE-A-37 30 022, using a Ziegler-Natta catalystsystem. The propylene copolymers can be prepared, for example, in agas-phase polymerization process at temperatures from 20 to 160° C. andat a pressure of from 1 to 100 bar. The molecular weights of thepolymers can be regulated by generally known measures, for example usingregulators such as, for example, hydrogen.

Examples of suitable comonomers are C₂ - and C₄ - to C₁₂ -α-monoolefins,especially C₂ - and C₄ - to C₆ -α-monoolefins, such as ethane, 1-butene,4-methyl-1-pentene, 1-hexene, n-1-octene, n-1-decene and n-1-dodecene.

Random polypropylene copolymers which are particularly suitable formentioning in this context are those comprising from 1 to 4% by weightof ethylene and from 99 to 96% by weight of propylene, based in eachcase on the overall weight of the monomer composition, the randomcopolymers having a molar mass distribution M_(w) :M_(n) in the rangefrom 3 to 6 and a melt index MFI 230° C./2.16 kg in the range from 5 to9 g/10 min (measured in accordance with DIN 53 735). These polypropylenecopolymers have a melting range of from about 135 to 155° C. (determinedby DSC). When plastics films based on such polypropylenes are used,plastics film/metal laminates are obtained which show no white fracture.

Particular preference is given, furthermore, to random polypropylenecopolymers comprising from 90 to 97% by weight propylene, from 2 to 5%by weight of ethylene and 1 to 6% by weight of 1-butene, based in eachcase on the overall weight of the monomers, and having a molar massdistribution M_(w) :M_(n) in the range from 3 to 6 and a melt index MFI230° C./2.16 kg in the range from 4 to 8 g/10 min. Such randompolypropylene copolymers have a melting range from 120° C. to 140° C.(determined by DSC). The plastics films which are obtainable from thesepolypropylene lead to plastics film/metal laminates which likewise showno tendency whatsoever toward white fracture.

All stated values for the melt index MFI relate to the measurement inaccordance with DIN 53735.

The polypropylene copolymers used to produce the novel plastic/metalcomposites are obtainable, for example, under the trade name Novolen®3225 MCX and Novolen® 3520 LX from BASF AG.

It is of course also possible to coextrude mixtures of theabovementioned polypropylene copolymers, preferably 1:1 mixtures, toform a film.

In accordance with the present invention the random polypropylenecopolymer used to prepare the polypropylene film can be replaced in aproportion of up to 50% by weight by polypropylene homopolymer. In thiscase, therefore, a mixture of random polypropylene copolymer and ofpolypropylene homopolymer is extruded to form a polypropylene film. Ifmore than 50% by weight of polypropylene homopolymer is used in theplastics mixture, the metal/plastics laminates produced therefrom show acertain tendency toward white fracture. Suitable propylene homopolymershave a molar mass distribution M_(w) :M_(n) in the range from 2 to 10, amelt index MFI 230° C./2.16 kg in the range of from 1 to 20 g/10 min(measured in accordance with DIN 53 735) and an isotactic index in therange from 80 to 99%, preferably from 90 to 98%.

Preference is given to employing a mixture of the random polypropylenecopolymer described and of a polypropylene homopolymer having a molarmass distribution M_(w) :M_(n) in the range from 3 to 6 and a melt indexMFI 230° C./2.16 kg in the range from 4 to 15 g/10 min (measured inaccordance with DIN 53 735). The isotactic index of these polypropylenehomopolymer is in the range from 80 to 99%, preferably in the range from90 to 98%. The homopolymers are known and can be prepared, for example,by the polymerization process described in DE-A-3730022.

Particularly preferred propylene homopolymer are those having a molarmass distribution M_(w) :M_(n) in the range from 3 to 5 and a melt indexMFI 230° C./2.16 kg in the range from 10 to 12 g/10 min (DIN 53 735).

Suitable propylene homopolymers are obtainable, for example, under thetrade name Novolen® 1100 N and Novolan® 1125 N (BASF AG).

The thermoplastic polypropylene plastics films described can alsocontain customary additives, for example internal and externallubricants, antiblocking agents, stabilizers, antioxidants, pigments,crystallization auxiliaries and the like. These additives are employedin the amounts necessary for preparation, processing, finishing and use,in the form of coarse or fine powders or beads or are incorporateddirectly in the polymer. Further details regarding the commonly usedamounts and examples of suitable additives can be found, for example, inthe book Gachter-Muller, Kunstutoff-additive [Plastics additives],Carl-Hanser Verlag.

It is particularly advantageous if the thermoplastic polypropylene filmscontain up to 0.5% by weight, based on the overall weight of the film,of erucamide and/or oleamide as lubricants and up to 0.2% by weight,based on the overall weight of the plastics film, of antiblocking agent,preferably silica, and also, if desired, antioxidants and, if desired,further processing stabilizers.

Antioxidants used are preferably phenol derivatives. Further suitableadditives are titanium dioxide, calcium carbonate, diatomaceous earth,calcium stearate, and primary and secondary fatty acid amides. UVstabilizers employed, for example, are UV stabilizers of the HALS type.

The layer of adhesion promoter arranged between the polypropylenesupport film and the metal can likewise contain the abovementionedadditives. Preferably, however, they are incorporated into thepolypropylene support film.

The production of the plastics film/adhesion promoter/metal compositesis a generally known process. The procedure for it is first of all tocoextrude the thermoplastic for the support film, and the adhesionpromoter, together. The metal sheet in then covered with the preparedcoex film in such a way that the layer of adhesion promoter in thecomposite contacts the metal surface. Through the application ofpressure and heat, the polypropylene film/adhesion promoter/metalcomposite is prepared either by means of a heatable press or in the rollnip of a roller assembly or calendar by means of heatable rollers. Thepressure and the temperature here are to be chosen such that, on the onehand, the adhesion promoter enters into a solid and stable bond with themetal film and/or the metal sheet, and, on the other hand, thethermoplastic layer does not melt.

The coating of the metal sheet, and/or the thermoplastic composite film,generally has an overall dry-film thickness of less than 500 μm,preferably from 10 to 200 μm and, with particular preference, of leasthan 100 μm. The thickness of the adhesion-promoter layer is between 0.5and 100 μm. Correspondingly, the thickness of the polypropylene filmlayer works out at values between 10 and 499.5 μm. As already mentioned,it is possible to employ thermoplastic composite films consisting solelyof an adhesion promoter layer and a top layer, although composite filmscomprising a plurality of layers can also be employed. In this case, thevarious thermoplastic layers can each consist of identical or elsedifferent materials in an identical or different layer thickness.

In conclusion it should also be mentioned that it is also possible togive the sheet metal, on the side facing the contents, a coating aswell, with a preferably planar, thermoplastic composite film or elsewith a liquid or pulverulent coating material.

The polypropylene top layer/adhesion promoter/metal composites producedin the manner described are employed for the production of packagingcontainers, and in particular for the production of bases and/or lids ofcans, valve plates of aerosol cans, and closures. The preparation of theclosure components is carried out by the customary methods (cf., forexample, VR-INTERPACK 1969, pages 600-606: W. Panknin, A. Breuer, M.Sodeik, "Abstreckziehen als Verfahren zuw Herstellen von Dosen ausWeiβblech" [Drawing and wall ironing as a method of producing tinplatecans], SHEET METAL INDUSTRIES, August 1976: W. Panknin, C H. Schneider,M. Sodeik, "Plastic Deformation of Tinplate in Can Manufacturing";Verpackungs-Rundechau, Issue 4/1971, pages 450-458: M. Sodeik, I.Siewert, "Die nahtlose Dose aus Weiβblech" [The seamless tinplate can];Verpackungs-Rundechau, Issue 11/1975, pages 1402-1407: M. Sodeik, K.Haaβ, I. Siewert, "Heratellen von Dosen aus Weiβblech durch Tiefzishen"[Manufacture of tinplate cans by deep drawing], Arbeitsmappe fur denVerpackungepraktiker, Metalle, Teil II, Gruppe 2, Weiβblech, Ser. No.220.042 to 220.048 in neue Verpackung 12/1987, page B 244 to B 246 andneue Verpackung 1/1988, pages B 247 to B 250).

For further details, therefore, refer to the literature.

The novel propylene film/adhesion promoter/metal composites showpractically no more white fracture; moreover, the bonds between themetal and the adhesion promoter on the one hand and between thepolypropylene film and the adhesion promoter on the other hand areextremely good. The polypropylene top layers protect the metal very wellagainst the attack of the contents, and effects on the contents as aresult of corrosion products of the metal are likewise prevented.Impairment to the contents by constituents which have leached out of thepolypropylene top layer film has not been found during sterilization andstorage of the packaged goods.

I claim:
 1. A powder coating for the coating of metal can containers,wherein1) the powder coating comprises:a) from 65 to 80% by weight,based on the overall weight of the powder coating, of a thermoplasticcomponent, wherein the thermoplastic component comprises at least onethermoplastic material, b) optionally, less than 20% by weight, based onthe overall weight of the powder coating, of at least one thermosetresin, and c) optionally, less than 15% by weight, based on the overallweight of the powder coating, of at least one compound selected from thegroup consisting of pigments, fillers, catalysts, degassing agents,leveling agents, free-radical scavengers, antioxidants, and mixturethereof, and 2) the powder coating has a particle-size distribution suchthat:a) at least 90 percent by mass of the powder-coating particles havea size of between 1 and 120 μm, b) the maximum size of thepowder-coating particles is ≦150 μm for at least 99 percent by mass ofthe particles, and c) the mean size of the powder-coating particles isbetween 1 and 60 μm.
 2. A powder coating as claimed in claim 1, whichhas a particle-size distribution such thata) at least 90 percent by massof the powder-coating particles have a size of between 1 to 40 μm, b)the maximum size of the powder-coating particles is ≦60 μm for at least99 percent by mass of the particles, and c) the mean size of thepowder-coating particles is between 2 and 12 μm.
 3. A powder coating asclaimed in claim 1, which has a particles-size distribution such thata)at least 90 percent by mass of the powder-coating particles have a sizeof between 5 and 25 μm, b) the maximum size of the powder-coatingparticles is ≦40 μm for at least 99 percent by mass of the particles,and c) the mean size of the powder-coating particles is between 2 and 12μm.
 4. A powder coating as in claim 1 wherein the at least onethermoplastic material is selected from the group consisting ofpolyurethanes, polyamides, polyethylenes and polyesters.
 5. A powdercoating as in claim 1 having a glass transition temperature of >40° C.6. A laminate for the production of packaging containers, comprising asubstrate, having thereon a powder coating as claimed in claim
 1. 7. Apowder coating composition according to claim 1 wherein the coatingcomposition comprises a thermoset resin.
 8. A powder coating compositionaccording to claim 7 wherein the thermoset resin is an epoxy resinhaving an equivalent weight of from 400 to
 3000. 9. A powder coating asin claim 1 wherein the thermoplastic material comprises a polyurethane.10. A powder coating as in claim 9 wherein the thermoplasticpolyurethane has a Shore A hardness from 70 to 95, and a Shore Dhardness from 54 to
 90. 11. A powder coating as in claim 7 wherein thethermoset resin comprises at least one epoxy resin.
 12. A powder coatingas in claim 11 wherein the epoxy resin is present in the powder coatingin an amount from 10 to 15% by weight, based on the overall weight ofthe powder coating.